In a medium saucepan, combine the sweetened condensed milk, butter, and salt.
Cook over medium heat, stirring constantly, until the mixture thickens and becomes creamy, about 10-15 minutes.
Remove from heat and stir in the coconut until well combined.
Pour the mixture into a parchment-lined 8x8 inch baking pan.
Refrigerate for at least 2 hours, or until firm.
Cut into squares and serve.
